Hormones & Chemical Signaling
Lecture Outline
• Communication Basics
– Communication Overview
– Communication Methods
– Signal pathways
• Types
• Regulation (modulation) of signal pathways
– Homeostasis . . . again
• Endocrine System
– Hormones
• what they are
• How they work
Communication Basics: Overview
• Physiological Signaling (Communication) occurs via
– Electrical signals
• Changes in a cells membrane potential
– Chemical signals
• Molecules that are secreted into the ECF
• Responsible for most communication
– Target cells
• Those cells that receive the message regardless of its chemical
or electrical nature
Communication Basics: Methods
• Four methods of cell communication
1.
2.
3.
4.
Gap junctions
Contact – dependent signals
Local communication
Long distance communication
Communication: Gap Junctions
• Recall Structure…
• Function as a result:
– Controllable
• Open vs. closed states
– Passage of small
molecules:
•
•
•
•
Amino acids
ATP
cAMP/cGMP
Ions
– Allows tissues to work
as a syncytium
Communication:
Contact Dependent Signals
• Exactly what it sounds like:
– Two cells contact each other and…
• some types of immune responses can start
• cells know where they are…
– neurons during growth and development
• platelets can do their thing
– CAMs can act as receptors/signalers
• Via linkage to intracellular components
– Cytoskeletal structures
– enzymes
Communication:
Contact Dependent Signals
P-selectin – stored inside cells when not
needed
When inserted into the cell membrane,
contact by leukocytes causes their
recruitment
Integrins – shown to be involved in
contact signaling between platelets
Communication: Autocrine Signaling
• Cell – to – Self Cell
– Cell secretes chemical in response to stimulus
– Chemical binds to receptor on its own membrane
– Examples:
Chemical
Source
Target
Effect
IL-1
Macrophage
Macrophage
Inflammation
IL-1
B-cell
B-cell
Maturation &
proliferation
IL-6
B-cell
B-cell
Differentiation into
plasma cells
Communication: Paracrine Signaling
• Chemical Signals secreted and effect
neighboring cells
– Some signals act as paracrine & autocrine
messengers
– Include classes of chemicals such as cytokines &
eicosanoids (prostaglandins, prostacyclins,
thromboxane and leukotrienes)
– Ex. Histamine – belongs to cytokine group
• Acts on local area cells, they increase p-selectin membrane
molecules which attract leukocytes.
• Can cause phosphorylation of CAM molecules, which causes
increased cellular separation… making them leaky!
Communication: Cytokines
• Chemical messenger hybrids
– Act as paracrine messengers as well as long
distance messengers, but…
• Not hormones because they work on many
different cells
• Not produced by an endocrine gland
Communication: Long Distance
• Long Distance communication occurs by
– Electrical signaling (action potentials)
– Endocrine System – Hormones:
• chemical messengers secreted by glands into the blood
• not specific in where they go
• specificity is due to the receptors!
– Nervous System
• Chemicals released due to electrical signal and becomes:
– neurocrines – released and binds to target at the immediate area
» Ex. Acetylcholine, GABA
– neurohormones – released into the blood
» Ex. Antidiuretic hormone & oxytocin
Communication: Long Distance
Communication: Long Distance
(neurocrines)
Communication: Signal Pathways
• How do hormones create a reaction in
some cells and not others?
– The receptor proteins
• If a receptor is present, the effect of
binding always initiates a response
through a signal pathway
Signal
molecule
binds
to
Receptor activates
protein
Intracellular
alters
signal
molecules
Target creates
proteins
Response
Communication: Signal Pathways
• Receptor Protein
Location
– Intracellular
• Chemical messengers
must be lipophilic
• Bind to cytosolic
receptors or nuclear
receptors
– Effect is to modulate
gene activity (+ or -)
– Cell Membrane
• Lipophobic molecules
bind to membrane
receptor
• Receptor transfers the
signal to the ICF (signal
transduction)
Communication: Signal Transduction
Available Options for Signal Transduction
Communication: Signal Transduction
• Why do we care
about signal
transduction?
– Another
example of big
payoff with little
effort!
– Amplification
Communication: Signal Transduction
• Signal Transduction & amplification relies on the following
process:
1
1. Molecule (primary messenger) in ECF binds to membrane receptor
and activates it
2. Membrane proteins are activated which may
a. Activate protein kinases
b. Activate enzymes that create secondary messengers
10
3. Secondary messengers
a. Alter ion channel gating resulting in membrane potential change
b. Increase intracellular calcium
c. Alter enzyme activity of protein kinases (phosphatases)
1000
4. Proteins modification (by Ca2+ or PO4-) affects
100,000
a.
b.
c.
d.
Metabolic enzymes
Motor proteins
Gene expression (and therefore protein synthesis)
Membrane transport & receptor proteins
Communication: Signal Transduction
Same Steps (summary)
1. Molecule (primary
messenger) in ECF
binds to membrane
receptor and
activates it
2. Membrane proteins
are activated which
may
3. Secondary
messengers
4. Proteins modification
(by Ca2+ or PO4-)
affects
Communication: Signal Transduction
• This pathway is a cascading event
What kind of mechanism
are cascading events?
MANY PRODUCTS!!
Communication: Signal Transduction
• Channel receptors
– Ligand binds and
electrical signal is
formed
– Creates a very
fast intracellular
response
– May open via
other pathways as
well
Communication: Signal Transduction
• Receptor-Enzyme and Signal Transduction
– Binding of ligand causes activation of the active binding
site on enzyme and are either
• Protein kinases
– transfer phosphates
• Guanylyl cyclase
– converts GTP to cGMP
(2 messenger)
– Insulin, cytokines and
growth factors bind to
receptor enzyme
complexes
Communication: Signal Transduction
• G-Protein Activation (G proteins on front cover)
– Most common signal transduction pathway
• Receptor (G protein-coupled receptor) is linked to
a G protein (ICF peripheral protein) transducer
molecule
• Activated by exchange reaction (GDP to GTP) and
– Open ion channel OR
– Activate amplifier enzyme (most common pathway)
» Adenylyl cyclase and phospholipase C are the most
common amplifier enzymes
Communication: Signal Transduction
• G protein-coupled adenylyl cyclase-cAMP system
– Process figured out in the 1950s by Earl Sutherland and
subsequently won a Nobel prize for it!
– Most commonly used for protein hormones
Communication: Signal Transduction
• G protein-coupled phospholipase C system
– When activated G protein activated phospholipase C,
it converts a membrane phospholipid (phosphatidyl
inositol bisphosphate) into diacylglycerol (DAG) and
inositol trisphosophate (IP3)
– DAG is non polar and remains in the phospholipid
bilayer where it activates protein kinase C (PK-C)
• PK-C phosphorylates cytosolic proteins and furthers the
cascade effect
– IP3 is hydrophilic and enters into the cytosol where it
binds to ER and opens Ca2+ channels and acts as a
signaling molecule
Communication: Signal Transduction
Communication: Signal Transduction
Communication: Signal Transduction
• Integrin Receptor Signal Transduction
– Integrins are membrane spanning proteins
involved in
•
•
•
•
•
Hemostasis
Tissue repair
Cell adhesion
Immune processes
Cell migration during development
Communication: Signal Transduction
• Integrin Receptor Signal Transduction
– When ligand binds to integrin
• Intracellular enzymes are activated and
• Cytoskeletal organization is changed
• Quite a few pathways figured out
• One important one is when an integrin membrane
receptor is missing and platelet activation does not
occur = hemophilia
Communication: Signal Transduction
• LOL!
Communication: Signal Transduction
Over-view
Communication: novel signal molecules
• Intracellular signal molecules
– Ca2+, NO, CO, H2S and
– Two important eicosanoids derived from
arachadonic acid
• Leukotrienes
• Prostanoids (prostaglandins & thromboxanes)
Communication: novel signal molecules
• Effects of Ca2+
when intracellular
levels increase
Communication: novel signal molecules
• NO, CO and H2S
– Short acting paracrine/autocrine signal molecules
– NO acts as a vasodilator by diffusing from the cell
that produced it into the surrounding tisse
• Activates formation of cGMP which can block channels,
causing muscle to relax
– CO known for its affinity for hemoglobin (thus
starving tissues of oxygen) it also
• Activates formation of cGMP
– H2S also acts as a vasodilator
• Garlic is a good supply of sulfur compounds…
Communication: novel signal molecules
• Lipids as paracrine signal molecules
– Derived from arachidonic acid (precursor to
eicosanoids)
– Phospholipase A2 is responsible for the
production of arachidonic acid
• Arachidonic acid can act as a secondary
messenger by
– Influencing ion channels & Intracellular enzymes
• Arachidonic acid may also produce two other
paracrine messengers
– Leukotrienes
– Prostanoids (prostaglandins and thromboxanes)
Communication: novel signal molecules
• Leukotrienes
– Secreted by some leukocytes
– Initiate smooth muscle spasms in bronchioles
– Also involved in anaphylaxis
• Death unless medical intervention
• Prostanoids
– Produced as a result of cyclooxygenase (COX) action on
arachidonic acid
• Products are prostaglandins and thromboxanes
– Influence sleep, inflammation, pain, fever
• Cox inhibitors (aspirin, ibuprofen)… stop the formation of
prostaglandins = stop the pain!
• Sphingolipids – also be involved with G protein coupled
receptors
Communication: Modulation of Pathways
• How are these pathways controlled?
– Receptors are proteins!
• Subject to
– Specificity of binding
– Competition for binding site
» Agonists and antagonists
– Saturation of ligand
» Up regulation and down regulation of receptors
– Pathways are mechanisms under
homeostasis guidelines
Communication: Modulation of Pathways
• Continued Tuesday…